1. Field of the Invention
The invention relates generally to a means and method for monitoring load conditions on three-phase motors, and particularly to a means and method utilizing solid state overload control circuits for monitoring the load on each of the three phases of the motor while permitting for external calibration of the overload current gain and the trip delay sequence.
2. Description of the Prior Art
Historically, three-phase motor overload protection circuits have been designed to protect electrical motors and connected loads against potential damage caused by thermal overload and cooling problems, mechanical overload and electrical fault conditions.
In recent years, changes in electric motor design and manufacturing techniques have led to the introduction of a basic motor architecture where general purpose motor applications no longer accept a constant overload of 15-20% beyond design capacity without overheating and potentially causing thermal damage to insulation or mechanical construction. Because of these design changes, the newer motors have a faster rate of temperature rise and a much lower ability to withstand even a low level of continuous overload. These new design parameters require greater accuracy and better time/current characteristics (thermal modeling) from the protection overload circuitry.
The primary function of any overload circuitry is to provide an overload relay to switch off the motor when it begins to draw more current than the rated full load amperage (FLA). In the past, this was done by passing the motor current through a bimetallic or eutectic alloy sensing element which would heat up and trip if the current rating was exceeded. The trip point of these devices was normally plotted on what has now become a standard motor time/current trip curve. For example, if a 600% motor current was passed through an overload device, it would trip within a specific period of time. If a 400% current was passed, the overload device would trip at a different time. The actual trip time depended on the heater coils installed in the device.
The curve attempts to duplicate the thermal model of each motor in order to achieve a trip time in advance of when the motor absorbs the amount of heat required to do thermal damage to the windings. Typically, motor manufacturers assign a specific thermal rating to their motors to indicate how much heat the motor can tolerate and for how long.
While each thermal element in the thermal type overload relay could be calibrated to an individual motor, they were still affected by the external surrounding air temperature which was not always the same as the ambient temperature associated with the motor. Also, because of the difference in mass of the thermal element and the motor, the overload relay would cool down faster than the motor and subsequently be able to reset before the motor had sufficiently cooled. Successive overload trips caused by this consistency would decrease motor life.
Another problem was that motors with similar horsepower ratings, from different manufacturers, had varying thermal capacities although they met the same rating specification.
To overcome these difficulties, electric overload relays were offered as alternatives to the thermal overload device. The early attempts at direct conversion of the thermal overload to an electronic model offered only limited protection. They also introduced a number of drawbacks without providing the total economic solution.
Auxiliary voltage supplies were needed to power the electronics and, in addition, either special transducers or current transformers (CTs) were required to provide an isolated measurement signal. In addition, analog devices were not able to accurately model the motor's time/current curve (thermal model).
More recently, electronic techniques eliminating the need for an auxiliary power source and incorporating integral CTs have provided motor protection relays which are far more accurate yet easier to install than the early thermal overload relays. By using digital technology, closer thermal modeling is possible for very accurate motor protection. It is also possible to provide greater overall system protection, including in addition to overload indication, auxiliary control and access information contained within the motor and control circuit system.
An example of a currently available digital motor protection circuit is the C311 digital motor protection sytem offered by Cutler-Hammer, a division of Eaton Corporation, the assignee of the subject invention.
While digital electronic motor protection devices are becoming the norm and are recognized to provide superior motor protection by accurately modeling the time/current curve (thermal capacity) of the motor, there are several drawbacks to the digital overload protection circuits currently available. Such devices are almost cost-prohibitive on lower level applications, making their superior monitoring features unavailable in certain applications. Those inexpensive digital systems which are currently available sacrifice some of the very features which make electronic overload circuit protection desirable. For example, to achieve greater cost efficiency, less expensive digital overload protection circuits often monitor two phases of a three-phase motor, ignoring fault conditions which may be present on the other phase. In addition, many of the low-to-mid range electronic protection circuits are void of metered calibration devices, requiring the user to guess and set the overload monitoring parameters by the trial and error method.
The solid-state overload relay protection circuit of the subject invention is designed to provide a low-cost, efficient means and method for providing overload protection while preserving the features of more expensive devices now available, including the recognized improvements over either eutectic or bimetal overload relays.